mGCA: We don't properly check associated const bindings for well-formedness

[fmease]

Uplifted from #150843 (comment), #150843 (comment). Follow links for further details. Credits to @BoxyUwU.

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Example reproducers:

RHS is a const param.

//@ known-bug: unknown
//@ check-pass

#![feature(min_generic_const_args)]
#![expect(incomplete_features)]

trait Trait { #[type_const] const CT: bool; }

fn f<const N: i32>(_: impl Trait<CT = { N }>) {} // FIXME: this should yield a type mismatch (`bool` v `i32`)

RHS is a const param.

//@ known-bug: unknown
//@ check-pass

#![feature(min_generic_const_args)]
#![expect(incomplete_features)]

trait Trait { #[type_const] const CT: bool; }

fn f<const N: i32>() {
    let _: dyn Trait<CT = { N }>; // FIXME: this should yield a type mismatch (`bool` v `i32`)
}

RHS is a (normalizable) const projection (that doesn't contain any generic params).

//@ known-bug: unknown
//@ check-pass

#![feature(min_generic_const_args)]
#![expect(incomplete_features)]

trait Trait { #[type_const] const CT: bool; }

trait Bound { #[type_const] const N: u32; }
impl Bound for () { #[type_const] const N: u32 = 0; }

fn f() { let _: dyn Trait<CT = { <() as Bound>::N }>; } // FIXME
fn g(_: impl Trait<CT = { <() as Bound>::N }>) {} // FIXME

[fmease]

We successfully detect mismatches if the RHS is a number literal, string literal, const tuple, const array, (unit, tuple, normal) struct literal in const { … }, path to free type const in const { … }, const projection in const { … } etc.

We fail to detect mismatches if the RHS is a const param, a (unit, tuple, normal) struct literal in { … }, path to free type const in { … }, const projection in { … } etc.

[BoxyUwU]

the dev guide chapter on const generics talks a little about the process by which we check const arguments have the right type. effectively the thing thats happening here is that some cases are anon consts (which check themselves kinda,) and other cases rely on ConstArgHasType (which we aren't requiring for assoc const bindings)

[zedddie]

@rustbot claim